1. Field of the Invention
The present invention relates to sol-gel derived silicon-containing monoliths which are crack-free and suitable for firing to produce dense ceramic shapes. The use of specific additives, drying control chemical additives (DCCA), during the mixing phase of sol-gel process makes possible the production of large scale fully dried crack-free monolithic gels routinely and rapidly.
2. Description of the Prior Art
The production of ceramic powders by the sol-gel process has been known for many years. For example, Thomas, U.S. Pat. No. 3,791,808, discloses a process for the production of a thermally crystallizable oxide product formed by hydrolyzing a silicon alkoxide with water in the presence of a catalytic amount of acid hydrolysis catalyst to prepare a clear solution of a partially hydrolyzed silicon alkoxide, reacting the partially hydrolyzed product to form a clear gel, heating the gel to substantially remove organic and free liquid components, and converting the gel to thermally crystallizable oxide product having a particle size of less then amount 0.2 microns.
Recently, this low temperature chemical processing method has been investigated for preparation of monolithic objects. And while there are many potential advamtages to sol-gel processing such as the ability to produce uniquely homogeneous solids of unique composition ranges, better purity, lower temperature of preparation, tighter grain size control, greater surface smoothness, better control of densities, and the potential for recycling defective gel products prior to firing, the non-homogeneous pores that are present in the solid after gelation usually lead to catastrophic failure of the piece during drying due to the development of large capillary stresses. These large capillary stresses occur when differential evaporation rates takes place from capillaries intersecting the surface. When the gel strength is low, the surface cannot withstand the non-homogeneous stress, thereby resulting in fracture of the article. Among the methods employed by the prior art in an effort to circumvent the cracking of the monolith during the drying process are (1) increasing the mechanical strength of the gel by aging, (2) diminishing the magnitude of capillary forces by enlarging the pore size and/or decreasing the surface energy by use of surfactacts, (3) reducing the rate of evaporation of the solvent from the pores by use of a semipermeable membrane during drying, (4) elimination of the pore liquid-solid interface by hypercritical evacuation, and (5) freeze drying. However, application of these very methods in rapid, routine production of large scale, fully dried monoliths is unreliable, uneconomical, and extremely difficult. None of these prior art methods have proven capable of meeting the current industry requirements for production of large objects in a short time frame.
Thus, a need has continued to exist for large sol-gel derived silicon-containing monoliths which can be quickly dried, aged, and densified to crack-free shaped articles and a method for producing these monoliths.